1. Field of the Invention
The invention relates to a method for producing a re-releasable bond between two wafers which is stable at high temperatures and is mechanically stable.
Modern semiconductor devices are increasingly produced on very thin semiconductor bodies, including ones that are thinner than 100 xcexcm. Typically used for this purpose are conventional wafers that have a thickness of 500-700 xcexcm and are ground until thin before the production of the semiconductor devices.
However, on account of their mechanical properties, such ultrathin wafers are very difficult to handle and therefore cannot be handled by the same production machines and transporting and securing devices as wafers of a standard thickness. It is therefore necessary to provide production machines and transporting devices which are modified specifically for ultrathin wafers, are configured for special wafer cassettes and have gripping devices for feeding the production machines that are configured specifically for ultrathin wafers and usually have to be operated manually. Furthermore, here the devices for fixing the ultrathin wafers during the actual production process, such as for example chucks, are converted for the requirements of the ultrathin wafers, which is laborious to various extents. There are, however, narrow limits to the modification of production machines for the purposes of handling ultrathin wafers, on account of their increasing complexity.
For all these reasons mentioned, the provision of new or modified production machines for the handling of ultrathin wafers is extremely complex and cost-intensive.
A significantly more simple method for producing and handling ultrathin wafers is described below.
A first wafer, known as the product wafer, on which the semiconductor devices are later to be applied and which therefore is to be ground until ultrathin, is placed and fastened in an interlocking manner onto a second wafer, known as the support wafer. The product wafer can then be ground until thin and the semiconductor devices can be introduced into the thin-ground product wafer using the corresponding production technology. Finally, the product wafer can be re-detached from the support wafer.
The interlocking and frictionally engaging bond is created here for example by a wax or adhesive bond or by a two-sided adhesive film disposed between the support wafer and the product wafer. The disadvantage of this method is that the interlocking and frictionally engaging bond is configured only for very low temperatures. At high temperatures, typical for the semiconductor process technology, the bond of wax, adhesive or film would however come apart again, that is to say a temperature-stable bond between the two wafers is no longer ensured here. In addition, if the materials mentioned above are used, foreign atoms could diffuse into the semiconductor body and contaminate it undesirably.
It is accordingly an object of the invention to provide a method for producing a stable bond between two wafers which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which is stable at high temperatures and is mechanically stable.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for producing a re-releasable bond being stable at high temperatures and being mechanically stable. The method includes the step of placing two wafers to be bonded one on top of another such that a surface of a first wafer is disposed on a surface of a second wafer, resulting in a creation of interspaces between the two wafers. The interspaces at least partially connecting the surface of the first wafer to the surface of the second wafer. A liquid glass compound is introduced into the interspaces forming a liquid glass film wetting inner surfaces of the interspaces. In the process, voids connected to an atmosphere surrounding the two wafers remain inside the interspaces wetted with the liquid glass compound. The two wafers lying one on top of the other are subjected to a temperature treatment to transform the liquid glass film into a solid silicon dioxide film.
The present method makes it possible for the two wafers to be bonded to each other according to the invention with a bond which is mechanically stable and is stable at high temperatures in a very simple but nevertheless effective way such that a product wafer as is known can subsequently be ground until thin and further processed. For the further processing of the product wafer, all the equipment of conventional silicon technology can be advantageously used, that is to say no additionally modified production machines or transporting devices for handling the thin-ground wafers are required here. The special advantage of the present invention is that the wafers bonded to each other by a thin silicon dioxide film can be detached from each other again very simplyxe2x80x94for example by a hydrofluoric acid compound.
In accordance with an added mode of the invention, there is the step of carrying out the temperature treatment step for turning the liquid glass film into the solid silicon dioxide film at a temperature of between 120xc2x0 C. and 450xc2x0 C.
In accordance with an additional mode of the invention, there is the step of subjecting the two wafers lying one on top of the other to a uniform pressure over a large surface area during the temperature treatment step.
In accordance with another mode of the invention, there is the step of using the first wafer as a support for the second wafer. The second wafer, on which semiconductor devices to be created are provided, is ground until thin or etched until thin after the production of the bond.
In accordance with a further mode of the invention, there is the step of forming trenches running in a plane of one of the surface of the first wafer and the surface of the second wafer. The trenches can be formed such that they run at least one of parallel and perpendicular to one another. The trenches have a depth of from 2 xcexcm to 10 xcexcm and are disposed at a distance of from 1 xcexcm to 10 xcexcm from one another.
In accordance with another added mode of the invention, there is the step forming the trenches to have a cross section being a rectangular cross section or a trapezoidal cross section.
In accordance with another additional mode of the invention, there is the step of disposing a multiplicity of fibers between the two wafers, and regions between neighboring fibers forming the interspaces.
In accordance with a further added mode of the invention, there are the steps of applying a layer of the liquid glass compound to one of the two wafers; subsequently placing the two wafers one on top of the other in an interlocking and frictionally engaging manner to create the liquid glass film; and spinning off excess liquid glass compound located in the interspaces by high-speed rotation of the two wafers, utilizing a centrifugal force.
In accordance with an added mode of the invention, there are the steps of placing the two wafers one on top of the other in an interlocking and frictionally engaging manner; disposing the two wafers in a liquid glass reservoir such that the liquid glass compound can penetrate into the interspaces, utilizing capillary suction; and spinning off excess liquid glass compound located in the interspaces by high-speed rotation of the two wafers, utilizing centrifugal force to create the liquid glass film.
In accordance with another mode of the invention, there are the steps of forming a through hole in at least one of the two wafers; placing the two wafers one on top of the other in an interlocking and frictionally engaging manner; spinning off the liquid glass compound through the through hole into the interspaces between the two wafers; and spinning off excess liquid glass compound located in the interspaces by high-speed rotation of the two wafers, utilizing centrifugal force, to create the liquid glass film.
In accordance with another mode of the invention, there is the step of placing a fiber mat produced from a fibrous material between the two wafers, regions between fibers of the fiber mat forming the interspaces.
In accordance with a further mode of the invention, there are the steps of performing one of impregnating and wetting the fiber mat with the liquid glass compound before the fiber mat is placed between the two wafers; subjecting the fiber mat impregnated with the liquid glass compound to a high-speed rotational movement normal to its plane, utilizing centrifugal force to adjust a film thickness of the fiber mat; and subsequently, placing the fiber mat in an interlocking and frictionally engaging manner between the two wafers.
In accordance with a another added mode of the invention, there is the step of storing the fiber mat impregnated with the liquid glass compound under refrigerated conditions until the temperature treatment step to create the silicon dioxide film is performed.
In accordance with another additional mode of the invention, there are the steps of impregnating the fiber mat with the liquid glass compound at an elevated temperature; spinning the liquid glass compound onto the wafers at an elevated temperature; and spinning off excess liquid glass compound at an elevated temperature.
In accordance with a concomitant mode of the invention, there is the step of using at least one of quartz glass fibers and carbon fibers as the fibrous material of the fibers of the fiber mat.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for producing a stable bond between two wafers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.